1. Field of the Invention
The present invention relates to a method of manufacturing a connecting rod for use in internal combustion engines, and more particularly to a method of manufacturing a connecting rod having a larger end which comprises a semicircular saddle joined to a shank and a semicircular cap that is held against and coupled to the semicircular saddle through respective mating surfaces.
2. Description of the Related Art
Connecting rods of internal combustion engines comprise a smaller-diameter end for connection to a piston, a shank joined at one end thereof to the smaller-diameter end, and a larger-diameter end joined to the other end of the shank for connection to a crankshaft. The larger-diameter end has a semicircular saddle joined to the shank and a semicircular cap that is held against the semicircular saddle. The saddle and the cap have respective mating surfaces engaging each other, defining a circular crank opening which receives the crankshaft therein.
If the mating surfaces of the saddle and the cap are completely flat, then the cap held against the saddle tends to be shifted in position with respect to the saddle due to vibrations, allowing stresses to concentrate on a certain local region of the saddle and the cap.
The completely flat mating surfaces of the saddle and the cap are liable to prevent the saddle and the cap from being positioned accurately with respect to each other when they are assembled together, and may possibly lead to an inadvertent error in combining the saddle and the cap in wrong orientations. One conventional way of solving these problems has been to attach a positioning pin to one of the saddle and the cap and a hole defined in the other, so that the saddle and the cap will be positioned accurately with respect to each other and combined together in proper directions with the positioning pin received in the hole. However, the positioning pin adds to the number of parts required, and the entire number of steps of the manufacturing process is increased because the positioning pin is attached and the hole is defined.
According to other prior solutions disclosed in Japanese laid-open patent publications Nos. 7-71438 and 7-100576 and U.S. Pat. No. 3,994,054, interfitting surface irregularities are formed on the mating surfaces of the saddle and the cap.
Specifically, Japanese laid-open patent publication No. 7-71438 reveals a manufacturing process in which a connecting rod shank preform and a cap preform are set in a forging die with their mating surfaces facing each other, and then simultaneously forged to form interfitting surface irregularities on the mating surfaces due to a material flow developed while they are being forged.
According to the process disclosed in Japanese laid-open patent publication No. 7-100576, a connecting rod shank blank and a cap blank are separately extruded such that they will have mating surfaces including interfitting surface irregularities. These blanks are cut to a shank and a cap, which are then held against each other at their mating surfaces and then forged.
The method shown in U.S. Pat. No. 3,994,054 first forms an integral connecting rod having a smaller-diameter end, a shank, and a larger-diameter end by forging. Then, cracking openings having notches are defined respectively in opposite flange portions at the center of the larger-diameter end. Thereafter, tapered cracking pins are forced into the respective cracking openings preferably with an impact force, cracking the larger-diameter end into a saddle and a cap with surface irregularities on their mating surfaces.
The manufacturing process shown in Japanese laid-open patent publication No. 7-71438 is disadvantageous in that the shank preform and the cap preform need to be fabricated separately and it is cumbersome and time-consuming to set these preforms in the forging die.
Problems of the process disclosed in Japanese laid-open patent publication No. 7-100576 are that it is necessary to form the shank blank and the cap blank separately, and an extra step is needed to cut the shank blank and the cap blank to a shank and a cap, respectively.
According to the method disclosed in U.S. Pat. No. 3,994,054, surface irregularities may not necessarily be formed on the mating surfaces of the saddle and the cap. In the absence of surface irregularities on the mating surfaces, the saddle and the cap may possibly be shifted in position relative to each other and combined with each other in wrong orientations when they are assembled together. Furthermore, since the connecting rod is forged, the roughness of circumferential edges around the cracking openings is so large that the circumferential edges will be strained to a large extent when it is cracked by the cracking pins.